Immobilization of Cr6+, Cd2+, Zn2+ and Pb2+ in alkali-activated slag binders

Granulated blast furnace slag is the main component of alkali-activated slag cementitious materials (AASCs). Calcium silicate hydrates with a low Ca/Si ratio, hydrotalcite-type phase, some amounts of hydrogarnets and sodium zeolites form as main AASC hydration products. The microstructure of alkali-activated slag pastes shows a higher amount of gel pore content compared to OPC pastes and, simultaneously, significantly lower amount of capillary pores. The microstructure and phase composition of hydrated slag indicate that they can play an essential role in the immobilization of heavy metals. The properties of alkali-activated slag pastes in the presence of Zn, Cd, Cr and Pb ions were studied. The leaching TANK test was used to evaluate the level of immobilization of particular elements in mortars made containing these elements. It was found that the degree of Cd, Zn and Pb ion immobilization was very high (exceeding 99.9%). The values for Cr6+ were lower (ca. 99.0%). The strength development as well as microstructure observations are presented in the paper.